Migrate from std::string to tensorflow::tstring.
Migrate from std::string to tensorflow::tstring.

Note that during the transition period tstring is typedef'ed to std::string.

See: https://github.com/tensorflow/community/pull/91
PiperOrigin-RevId: 281300599

google-research/seed_rl,google-research/seed_rl,google-research/seed_rl